If I don't join a high school network on facebook when I first created my account, can I still join one?

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I know that in the beginning you can type in your high school and get the confirmation of friend in that high school, but I skipped the step of adding a high school network much against facebook's advice mostly because I did not know what was going on. In facebook's Q&A, they specifically state you "cannot add high school networks to your account" under how many networks you can join question. I am currently in no network and I do not wish to join any college, work, or region networks because the first two don't apply to me and the last is too general. Does anyone know how to join a high school network if you chose not to join a high school network when they offered it to you in the beginning?